This invention relates generally to an implantable acetabular hip prosthesis, and more particularly to an implantable acetabular hip prosthesis which is well-suited for use in revision surgery or in other circumstances in which a void, such as a rim or cavitary defect, is present in the bony structure surrounding the natural acetabulum.
Total hip arthroplasty is a well-known and recognized procedure. A typical procedure involves replacing the top portion of the femur, including the head of the femur which serves as the "ball" in the ball-and-socket hip joint. Replacement of the socket is typically accomplished by cementing or otherwise fixing in a surgically prepared natural socket (the acetabulum) a substantially hemispherical cup or shell which is designed to receive the ballshaped head of the femoral prosthesis component. Fixation of the acetabular component within the surgically prepared natural acetabulum is vital to the long-term success of the procedure, and is typically accomplished by use of bone cement, bone screws, bone ingrowth processes, or a combination of these and/or other techniques.
A particular problem which orthopedic surgeons sometimes face in implantation of the acetabular component of a total hip prosthesis is the presence of a void or defect, such as a rim or cavitary defect, in the superior and superior/posterior acetabular region. Such voids or defects may be caused by superior or superior/posterior migration of a previously implanted acetabular prosthesis. Migration may also occur in situations in which a bipolar acetabular component has been implanted. Developmental problems (e.g., congenital dysplasia of the hip) may also produce voids or defects in the acetabular area. Natural deterioration of the bony tissue surrounding the acetabulum may also occur due to natural processes, such as infections, and/or trauma. Orthopedic surgeons presently use large segments of cadaveric bone graft in conjunction with known substantially hemispherical prosthesis components to reconstruct the acetabulum of a patient in which such voids or defects are present. However, this approach can be relatively time consuming and expensive, and does not always achieve desired and lasting results. Other attempts to address this problem, including the use of oblong acetabular components or alternative positioning of hemispherical components, have been made, but with only limited or mixed success.
It is an object of the present invention to provide an implantable acetabular hip prosthesis which is well-suited for use in the reconstruction of a natural acetabulum in which a superior or superior/posterior void or defect is present.
It is another object of the present invention to provide an implantable hip prosthesis which is particularly well-suited for use in the reconstruction of a natural acetabulum in which a superior or superior/posterior void or defect is present, and which is modular in design so as to allow the surgeon greater introoperative flexibility to properly reconstruct an acetabulum in which such a void or defect is present so that the center of motion of the hip prosthesis is very nearly the same as the normal center of motion of the hip.
These and other objects are achieved in an implantable acetabular hip prosthesis which comprises a primary shell having an outer surface adapted to be received within a surgically prepared acetabulum, and having an inner cavity for receiving a bearing insert which, in turn, receives a femoral head portion of a femoral component of a total hip prosthesis. The acetabular prosthesis further comprises an augmentation means, attachable to the primary shell, for filling a superior or superior/posterior defect, such as a rim or cavitary defect, adjacent the surgically prepared acetabulum. In one embodiment, the augmentation means comprises an augmentation piece, means for attaching the augmentation piece to the primary shell so as to allow the augmentation piece to be angularly (i.e., rotatably) adjusted relative to the shell, and means for fixedly retaining the augmentation piece in a selected angular orientation. The augmentation piece may comprise a partial sphere having a first surface adapted for implantation adjacent the bony tissue of the ilium (i.e., adjacent the surgically prepared periphery of the defect), a second surface adapted for disposition adjacent the outer surface of the primary shell, and a third surface which faces outwardly from the ilium when the hip prosthesis is positioned within the acetabulum. A plurality of holes extend from the third surface to the first surface for receiving bone screws to fasten the augmentation piece to the surrounding tissue. The third surface is provided with a recess to improve access to at least one of these screws.
In one embodiment of the hip prosthesis of the present invention, the means for attaching the augmentation piece to the primary shell, and for fixedly retaining the augmentation piece in a selected or desired angular orientation, comprises a Morse taper arrangement (which serves as the primary means of fixation) and a threaded screw located along a common axis which extends through the primary shell and the augmentation piece. The Morse taper arrangement preferably comprises a male Morse taper formed on the augmentation piece, and a mating female Morse taper formed in the primary shell. The head of the screw means is received within a recess formed within the inner cavity of the primary shell, and the body of the screw extends along the common axis through the Morse taper to engage threads which are formed in a threaded opening in the augmentation piece. The common axis preferably extends through the shell and the augmentation piece at an angle of approximately 45.degree. relative to the polar axis of the shell. In one embodiment of the invention, at least a portion of the outer surface of the primary shell and a portion of the third surface of the augmentation piece are porous coated to promote fixation to the adjacent bone by bone ingrowth processes.
In one embodiment of the invention, the augmentation means comprises a plurality of augmentation pieces of varying sizes and/or shapes, so as to allow the surgeon to select a piece which is most appropriate for filling the defect. This arrangement allows the surgeon some degree of flexibility in the operating theater to achieve a more precise fit, thus increasing the probability for the long-term success of the implant.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.